Manipulators, in particular manipulators such as industrial robots, are used for various work processes in, for example, assembly or production in industrial environments. An industrial robot is an automatically controlled, freely programmable manipulator, which can be programmed in three or more axes. Industrial robots can be mobile or can be employed at a fixed location.
An articulated-arm robot, or jointed-arm robot, is a three-dimensionally mobile industrial robot, which is made up of several pivot joints. Normally, the pivot joints are connected to arm links, and each pivot joint features a joint axis. Articulated-arm robots are often equipped with six joints, in order to represent each of the six possible Cartesian degrees of freedom with one joint axis if this is possible. If a robot has six or fewer degrees of freedom, the position and orientation of the individual arm links is usually clearly defined by the position of the Tool Center Point (TCP).
If, however, the manipulator has more than six degrees of freedom or more than six joint axes, the seventh axis and every additional axis represent an over-determination. With a stationary TCP, various configurations of the joints and arm links are therefore possible. The manipulator thus has additional degrees of freedom, which are considered to be redundant. Generally, redundant manipulators have more joint axes than Cartesian degrees of freedom. A manipulator having a redundant seventh axis advantageously features increased flexibility, because with a fixed TCP—i.e. when the position and orientation of the TCP in space remains unchanged—the manipulator can reach several poses. This increased flexibility, or flexible capability, can be made use of for example, for collision avoidance.
Nevertheless, a redundant manipulator can also reach undesirable poses. It is thus easy to imagine that, in the case of an articulated-arm robot having 7 joints, the redundant joint (usually the center joint or elbow joint) can realize a redundant movement. Thus this joint can, for example, due to a redundancy in an elbow joint of an articulated-arm robot, undesirably drop downwards or be displaced and prevent the execution of subsequent movements. This can occur in particular when the articulated-arm robot is being hand-guided in the force-compensated mode, in which, for example, the effect of gravity on the links of the manipulator is compensated for. Here, an elbow joint can, for example, drift into an inconvenient area and collide with its surrounding environment or with the operator. Furthermore, the articulated-arm robot can get into an unfavorable pose when being hand-guided, such that the robot can only be moved onwards by manual reorientation of the elbow angle. The operator must then disadvantageously use both hands to control the robot.
In the patent document WO 2014/043702 A1, published in the English language on Mar. 20, 2104 and hereby incorporated by reference in its entirety herein, a system and a method are described to limit the movement of a robot which has redundant degrees of freedom. In order to prevent the pose of a robot from changing in an uncontrolled manner when the robot is hand-guided in the force-compensated mode, it is proposed to limit the redundant degrees of freedom of the robot by applying restricting forces or torques internally to the joints, in order to force the robot arm into a preferred configuration. As a consequence, the mobility of the robot is generally limited.
Furthermore, it is known in the prior art to downgrade a redundant robot to a 6-axis kinematic system, for example by fixing or blocking the elbow joint. In addition, a reorientation of the elbow joint in the case of a fixed TCP (corresponds to a null space motion) for redundant robots is known. Nevertheless, no method exists to optimally control the behavior of an elbow joint while the robot is, in particular, hand-guided in the force-compensated mode.
In view of the above-described prior art, the objective of the present invention is to provide a method and a system which allow optimization of the behavior of the elbow joint of a manipulator in the hand-guided mode. In particular, this should prevent the elbow joint from moving freely, or from moving or being displaced beyond a certain position or beyond a certain angle, for example. Another objective of the present invention is to prevent a drifting of the pose of a redundant manipulator into an undesirable position. The objective is achieved with the method according to Claim 1 and the manipulator system according to Claim 10.